This disclosure relates generally to the field of interferometers, more specifically, to the construction and arrangement of a system and method of interferometers for use as spectrometers, such as Fourier transform spectrometers.
There are a wide range of applications for sensors that remotely detect the presence of chemicals and other materials including monitoring pollutants, monitoring climate change, and detection of toxins. One type of sensor, called a Fourier Transform Spectrometer (FTS), supplies spectral data for all these applications. The FTS comes in non-imaging and imaging variants that collect samples of the auto-correlation of the incoming light. The most common form of the FTS employs a Michelson interferometer with one variable length arm. Many variants of these exist yet they all require very precise control of the position of the variable arm, around 1/20th of a wavelength. The control system responsible for this precision can be the most expensive and least robust part of the system, especially for short wavelength (e.g. UV, VIS, NIR) sensors. These sensors all utilize the Discrete Fourier Transform (DFT) or its faster algorithm, the Fast Fourier Transform (FFT) to convert the auto-correlation (each spectral amplitude encoded as the amplitude of a cosine signal) to physical spectra. The FFT is efficient when large sample sets must be processed but produces artifacts if the variable arm is not precisely controlled, providing uniformly spaced samples. It also requires that the entire spectrum be computed at once despite the fact that most spectral targets can be detected using only a few spectral samples or subsets of the spectrum. The DFT/FFT have limited spectral range due to aliasing effects and once the band center is set in one part of the spectrum it cannot be modified elsewhere without recomputing the entire FFT. The spectra computed from DFT/FFT cannot independently change band centers, bandwidths, and line shapes.